VO4 tetrahedra

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

For each type of coordination number, the position of LMCT bands are sensitive to the presence of M" —O—M" linkages. This is found, for example, for species ranging from supported chromates to dichromates and polychromates [66, 67], and for isolated or polymeric VO4 tetrahedra present in Na3V04 and NH4VO3, respectively, which are considered as reference compounds for the interpretation of the spectra of supported species [68]. 

In diamine-templated compounds such as diaminobutane- (H2dbu) and diaminopropane-(H2dpr) templated uranyl vanadates, the aa/aa isomer allows sharing of two opposite equatorial edges of the interlayer UO7 bipyramids with four VO4 tetrahedra of the adjacent layers, so one interlayer uranyl ion is linked to four vanadate tetrahedra (Fig. 7e). Thus, the stmctural notation of these... 

D network of three U-polyhedra wide ribbons connected by VO4 tetrahedra I [I [I... 

Figure 8 (a)-The sheet anion topology in theM (U02)5(V0J205 compounds as an intergrowth between uranophane and P-UbO sheets. The orientations of the VO4 tetrahedra populating triangular sites lead to two different isomers (b and d) and to corrugated (c) or planar (e) layers for M = Na, K, P-Rb and M = a-Rb, respectively. 

An important difference between the structures of M6U5V2O23 (M = Na, K, P-Rb) and a-Rb6ll5V2023 concerns the orientation of the VO4 tetrahedra connecting successive ribbons. In the first series of the compounds, the V-0 bonds to the non-shared O atoms connecting two adjacent ribbons are all oriented in the same direction (Fig. 8b), leading to an angle of about 40°... 

This results in the formation of the i[U20io] chains with SBU consisting of UO7 and an UOe polyhedra connected through one oxygen (Fig. 12). These chains run along the a and b directions and are connected through VO4 tetrahedra. As in Li2(U02)3(V04)20, the UOe octahedra and VO4 tetrahedra form autunite anion-topology sheets parallel to (001) (Fig. 13a), but with an inverted occupation of the squares by the UOe octahedra and the V(2)04 tetrahedra. The V(l)04 tetrahedra are attached above and below the empty squares to form the [(U02)2(V04)3] double sheets (named D) Fig. 13b. 

Figure 11 The 3-D framework of mutually perpendicular [UO5] chains in Li2[(U02)3(V04)0] (a), where the autunite-type sheets of comer-sharing UO octahedra and VO4 tetrahedra (b) are...

Although the overall stmcture has a 3-D connectivity, as that of (U02)2(V207), it may be useful to consider it as based upon the 1 [UV209] layers of UO7 bipyramids and VO4 tetrahedra. The layers are parallel to (001) (Fig. 17b) and are further linked by divanadate groups along the c axis. 

Figure 10.6. A. Selection of V NMR spectra of thallium vanadates showing the effect of increasing distortion of the VO4 tetrahedra in progressing from TI3VO4 to TIVO3. The spectrum of TI2V6O16 arises from V in a distorted octahedral environment with almost axial symmetry. B. A series of NMR spectra of vanadates with V in both distorted tetrahedral and distorted octahedral environments. From Lapina e/a/. (1992), by permission of Elsevier Science.

Figure 6.28. Metal oxide layer, shown in two different orientations, in the crystal structure of tmaViOj, which contains VO5 pyramids and VO4 tetrahedra.

This peak is characterized by a reasonable tetrahedral configuration created by the oxygen atoms, except that it is too close to the existing V2. Considering the deficiency of the V2 site, it is feasible that the structure contains two types of [VO4] tetrahedra distributed in the structure in a random fashion. 

DR spectra in the UV-vis region of the undoped and doped samples are shown in Figure 2. In all cases, the presence of an absorption band at about 270-320 nm indicates the presence of isolated and/or polymeric VO4 tetrahedra [4,5]. Only in the case of the Bi(0.9) sample a second band can be seen at 475 nm, which could be related to the presence of species in octahedral coordination [12]. 

Crystal structures of sodium or ammonium metavanadate are composed of chain linkages of VO4 tetrahedra, 6 (20,21), similar to those of metaphosphates with high molecular weights. Anionic structures of... 

The alkylammonium intercalated VO, nanotubes are paramagnetic and show semimetallic conductivity probably due to the mixed valent V centers. The percentage content of V(iv) calculated on the basis of the effective magnetic moment is 45-50%. The structure contains VO4 tetrahedra and VO5 square-pyramids simultaneously. 

The polymeric metavanadate anions have been described above. Upon warming or ageing solutions that contain the decavanadate ion, it is possible to precipitate salts of the dark red trivanadate anion, MV3O8 (M = NH, K, Rb, Cs). The formula is sometimes written as M2V6O16, and the salts called hexavanadates in the literature. Potassium and cesium trivanadate contain infinite buckled layers of distorted VOs and VOe polyhedra. Another layer structure is found in yellow K3VSO14 which also is deposited from aqueous solution. The [VsOm] anion is composed of layers of comer-shared VO4 tetrahedra and VO5 square pyramids. ... 

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